Ruralmochaman The key issue is that if a device is compromised it can pretend to check the attestation chain and then display a message that everything is fine. If all that is displayed is "It's fine!" there's no way for a user to know when an "It's fine!" message is a lie.
To get around this, evidence must be checked by something that is believed to be uncompromised. The big QR code is the evidence. In theory it could be checked by hand using many manual calculations, but that would probably take a very careful person a year and a big stack of paper.